If you're using Vegas Pro, use a veg file to save a LOT of space and headaches :)
Do your first speed increase (Ctrl-Drag + Velocity envelope) to get the initial 18 hr. down to 1.5 hr.
Save this as a veg file.
Start a new project, import that veg file and repeat the process bringing you down to 7.5 minutes.
Save this as a new veg file, import it again and speed it up only 1.875 times to get down to 4 minutes.
Now render it.
NOTE: remember to disable resample to get rid of any ghosting.